Multi-layered optical disk reproducing method with focus search

ABSTRACT

To move an object lens out of focalization on a layer and to accelerate it upwardly, a drive signal DRV is set HIGH, and a focus OPEN signal is set LOW. After a value a counter exhibits at this time is held in a register T1, the counter is reset at zero. Then a value the counter exhibits upon FZC+ being LOW is held in a register T2 A , and a value the counter exhibits upon FZC- being HIGH is held in a register T2 B . The value of a register T3 is calculated from T1 and the average of T2 A  and T2 B , and when the counter value becomes equal to the sum of T1 and T3, DRV is set HIGH. After FZC- becomes LOW, OPEN is set HIGH, and the focus jump operation is completed.

This application is a division of application Ser. No. 08/571,360, filedDec. 13, 1995, now U.S. Pat. No. 5,754,507.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus and a method for reproducingdigital data, for example, recorded on two or more layers of amulti-layered disk.

2. Description of the Related Art

Optical disk reproducing apparatuses have been used for reading outdigital image data recorded on optical disks. Data recorded on a disk isdigital and contains a large amount of information. It often occurs,therefore, that the entirety of a desired unit of data cannot berecorded on a single disk. To cope with the problem, a multi-layereddisk having a plurality of recording layers on a single disk has beenproposed.

Upon reproducing data from an optical disk, the disk is rotated in apredetermined direction by a spindle motor after focus servo control andtracking servo control. Consequently, focus servo control is executed bymoving an object lens in a pickup in a face-to-face relation with thedisk so as to close the servo loop at the zero-cross of an S-shapedcurve of a first detected focus error.

If this is applied to a multi-layered optical disk having two or morelayers, in order to move focalization of the object lens from one layerto another, it is necessary to perform a focus servo control of adestined layer by conducting another focus search after canceling thefocus servo control which has been effective hitherto. During focussearch with a multi-layered optical disk, a number of S-curvescorresponding to the number of layers appear in the focus error signal.When the second layer is to be focalized, focus servo control may beperformed at the second S-curve. Similarly, for focalization to thefourth layer, focus servo control may be performed at the fourthS-curve.

Such focus search, however, takes a long time for movement from a layerto another.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an apparatus and amethod for reproducing data from a multi-layered disk, capable ofreducing the time required for movement from a layer to another.

According to the invention, there is provided a disk reproducingapparatus for reproducing data recorded on a plurality of layers of anoptical disk, comprising: first comparator means for comparing a focuserror signal with a first voltage; second comparator means for comparingthe focus error signal with a second voltage; drive voltage supplyingmeans for selectively supplying a first or a second object lens drivevoltage in response to results of comparison supplied from the firstand/or second comparator means; and means for driving an object lenswith the object lens drive voltage supplied from the drive voltage,supplying means.

There is further provided a disk reproducing method for reproducing datarecorded on a plurality of layers of an optical disk, comprising: afirst step for accelerating an object lens in a first direction; a:second step for detecting that a focus error signal surpasses a firstvoltage and for accelerating the object lens in a second directiondifferent from the first direction; a third step for detecting that theobject lens has moved for a predetermined duration of time and foraccelerating the object lens in the first direction; and a fourth stepfor detecting that the focus error signal surpasses a second voltage andfor starting focus servo control.

When focalization is changed from a layer to another of a multi-layereddisk, the object lens is first accelerated up to the rising of theS-curve in the focus error signal corresponding to the destined layer,passing over the just focus point, then accelerated in the oppositedirection, and again accelerated in the initial direction, thus toestablish focalization to the destined layer.

The above, and other, objects, features and advantages of the presentinvention will become readily apparent from the following detaileddescription thereof which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a multi-layered disk reproducing apparatusaccording to the invention;

FIGS. 2A to 2J are timing charts of a focus jump operation from a firstlayer to a second layer in the multi-layered disk reproducing apparatusaccording to the invention;

FIG. 3 is a flow chart of a soft-ware control of a focus jump operationfrom the first layer to the second layer in the multi-layered diskreproducing apparatus according to the invention;

FIGS. 4A to 4J are timing charts of a focus jump operation from thesecond layer to the first layer in the multi-layered disk reproducingapparatus according to the invention;

FIG. 5 is a flow chart of a soft-ware control of a focus jump operationfrom the second layer to the first layer in the multi-layered diskreproducing apparatus according to the invention; and

FIGS. 6A to 6J are timing charts of a focus jump operation from thefirst layer to the third layer of the multi-layered disk reproducingapparatus according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A multi-layered disk reproducing apparatus embodying the invention isdescribed below with reference to the drawings. FIG. 1 is a blockdiagram of the multi-layered optical disk reproducing apparatusaccording to the invention for reproducing data from an optical disk.Numeral 11 denotes a quartered detector comprising four detectors (11A,11B, 11C and 11D) which are based on an astigmatic process. Thequartered detector 11 detects whether an irradiated laser beam isexactly focalized on the optical disk. Detection signals of thedetectors 11A and 11C are supplied to an adder 12, and those of thedetectors of 11B and 11D to an adder 14. The sum signal (A+C) outputfrom the adder 12 is supplied to one of input terminals of an adder 20and one of input terminals of a comparator 16 through an amplifier 13.

The sum signal (B+D) output from the adder 14 is supplied to the otherterminal of the adder 20 and the other terminal of the subtracter 16 viaan amplifier 15. Supplied from the subtracter 16 is a ((A+C)-(B+D))signal as a focus error signal ERR which enters a phase compensationcircuit 17, one of input terminals of a comparator 22, and one of inputterminals of a comparator 25. Applied to the other terminal of thecomparator 22 is a comparison voltage V2. The comparison voltage V2 hasa value higher than noise near 0 V. The comparator 22 performszero-cross detection of the plus side, and outputs the detection signalto a switch 23 when the focus error signal ERR is equal to or higherthan the comparison voltage V2. Similarly, a comparison voltage V3 isapplied to the other input terminal of the comparator 25. The comparisonvoltage V3 has a value lower than noise near 0 V. The comparator 25performs zero-cross detection of the minus side, and outputs a detectionsignal to a switch 26 when the focus error signal ERR is equal to orlower than the comparison voltage V3.

The focus error signal, phase-compensated by the phase compensationcircuit 17, is supplied to one of selective terminals of a switch 18.Supplied to the other selective terminal of the switch 18 is a focussearch drive voltage from a switch 19. The switch 19 is supplied with afocus search drive voltage V+ at one of selective terminals and a focussearch drive voltage V- at the other selective terminal. When the outputsignal of the phase compensation circuit 17 is selected by the switch18, the focus servo loop for the optical disk is on. On the other hand,when the output signal of the switch 19 is selected, the focus servoloop is off, focus jump operation is done. In response to the signaloutput from the switch 19, an object lens provided in the pickup moves.When the focus search drive voltage V+ is selected at the switch 19, theobject lens is accelerated upward. When the focus search drive voltageV- is selected, the object lens is accelerated downward.

A reproduction RF signal (A+B+C+D) output from the adder 20 is suppliedto one of input terminals of a comparator 21. Applied to the other inputterminal of the comparator 21 is a comparison voltage V1. The comparator21 compares the reproduction RF signal with the comparison voltage V1.When the reproduction RF signal is equal to or higher than the,comparison voltage V1, a focus OK signal FOK indicating focalizationbeing proper is output to a switch 23 and a switch 26 as a controlsignal. When the switch 23 is turned on with the supply of the focus OKsignal FOK, a zero-cross detection signal FZC+ output from thecomparator 22 is supplied to a CPU 24. Similarly, when the switch 26 isturned on with the supply of the focus OK signal FOK, a zero-crossdetection signal FZC-output from the comparator 25 is supplied to theCPU 24. That is, switches 23 and 26 behave to gate the zero-crossdetection signals FZC+ and FZC- by using the focus OK signal FOK. Thisaims at preventing malfunctions by noise at ranges where thereproduction RF signal exhibits a low signal level. The CPU 24 is atypical one with a timer counter.

During focus jump operations, the focus search drive voltage V+ or V-supplied from the switch 19 is selected, and the selected drive voltageis output from the switch 18 to accelerate the object lens upward ordownward. Changeover of the switch 19 is controlled by a drive signalDRV from CPU 24 on the basis of the zero-cross detection signal FZC+ orFZC- supplied from the comparators 22 and 25. Focus search drivevoltages V+ and V- used in this example are equal in absolute value. InCPU 24, a focus OPEN signal is generated, and it is supplied to theswitch 18 as a control signal. Accordingly, the phase compensationcircuit 17 is selected by the switch 18. The signal selected by theswitch 18 is transferred through a drive amplifier 27 to a focus drivecoil 28 with one end grounded. The drive amplifier 27 is an N-timeamplifier for phase compensation.

When the focus OPEN signal becomes the HIGH level and the switch 18selects the phase compensation circuit 17, the focus servo loop is on.When the focus OPEN signal becomes the LOW level and the switch 18selects the switch 19, the focus servo loop is off, and the focus jumpoperation is performed.

FIGS. 2A to 2J are timing charts for focus jump from a first layer to asecond layer. At FIGS. 2A, taking time T on the horizontal axis and thelens height x on the vertical axis, the moving track of the object lensis illustrated. As stated above, for changing focalization from the justfocus point of the first layer to the just focus point of the secondlayer, the object lens is first moved to once overshoot the just focuspoint of the destined second layer, and thereafter moved back to befocalized at the just focus point of the second layer. The speed of theobject lens during these movements is shown at FIG. 2B where the lensspeed v lies on the vertical axis.

As shown at FIG. 2A, the movement from the just focus point of the firstlayer to the just focus point of the second layer starts at the point oftime t1. That is, at time t1, the control is changed from the focusservo loop control to the focus jump control. In the period T1 from timet1 to time t2, the object lens is accelerated upward. Time t2 is thepoint where the S-curve of the focus error signal ERR of the secondlayer, when rising, surpasses the comparison voltage V2. After theobject lens passes time t2, it is accelerated downward, passing a point(period of time t3) where the object lens intersects with the just focuspoint of the second layer. The period of time from the downwardacceleration of the object lens to the intersection of the object lenswith the just focus point of the second layer is designated by T2.

The object lens is further accelerated downward, and passes time t4where the lens speed is zero, taking the same period of time as T1 inwhich the object lens was accelerated upward. The period of time fromtime t3 where the object lens intersects the just focus point of thesecond layer to time t4 where the speed of the object lens becomes zerois determined as time (T1-T2). The object lens, after passing time t4,is accelerated downward by time t5 (in the period of time T3), and thencontrolled to stop at the just focus point of the second layer. Time t5is a point where the object lens is distant from the just focus point ofthe second layer by one half (x2) of such distance at time t4 (x1).

Then the distance x1 is

    x1=α(T1-T2).sup.2 /2                                 (1)

Similarly, the distance x2 is

    x2=αT3.sup.2 /2                                      (2)

where α is the acceleration.

CPU 24 calculates the point of time t5 (or the period of time T3) fromEquations (1) and (2).

    αT3.sup.2 /2=1/2×α(t1-t2).sup.2 /2       (3)

Equations (1) and (2) are related as shown by Equation (3) which can berewritten

    T3=(T1-T2)/√2                                       (4)

Among digital data recorded on the first layer of the optical disk, thereproduction RF signal is read out by time t1 as shown at FIG. 2C. Sincethe digital data recorded on the optical disk relies on the presence orabsence of pits, the reproduction RF signal exhibits the waveform asillustrated. In order to effect focus jump from the first layer to thesecond layer, the object lens must first be accelerated upward.Therefore, the focus OPEN signal (FIG. 2G) is set to the LOW level, andthe drive signal DRV (FIG. 2H) is set to the HIGH level. The focus OPENsignal shown at FIG. 2G maintains the LOW level to the end of thecontrol for focus jump.

When the drive signal DRV becomes the HIGH level, the object lens movesupward. Accordingly, the signal level of the reproduction RF signal islowered. When the reproduction RF signal becomes lower than thecomparison voltage V1, the focus OK signal FOK becomes LOW. When thesignal level of the reproduction RF signal again goes high and surpassesthe comparison voltage V1, the focus OK signal FOK becomes HIGH. This isbecause the focus jump causes the object lens to begin to move out offocalization on the first layer toward focalization on the second layer.After that, the focus jump to the second layer is completed at time t6,and digital data recorded on the second layer of the optical disk isread out.

If a focus error signal ERR (FIG. 2E) is generated in the minus side andsurpasses the comparison voltage V3, then the zero-cross detectionsignal FZC- shown at FIG. 2J becomes HIGH. When the focus error signalin the minus side approaches 0 V and becomes smaller than the comparisonvoltage V3, the zero-cross detection signal FZC- becomes LOW.

When the focus jump from the first layer to the second layer isperformed, the just focus point of, the second layer becomes nearer thanthe just focus point of the first layer. Therefore, a focus error signalERR is generated in the plus side with reference to the just focus pointof the second layer. As shown at FIG. 2E, when the focus error signalERR surpasses the comparison voltage V2 (time t2), the zero-crossdetection signal FZC+ becomes HIGH, and the drive signal DRV (FIG. 2H)becomes LOW. Therefore, the focus search drive voltage V- (FIG. 2F) issupplied to the focus drive coil 28. That is, the object lens isaccelerated downward. When the focus error signal ERR becomes lower thanthe comparison voltage V2, the zero-cross detection signal FZC+ becomesLOW.

The drive signal DRV (FIG. 2H) again becomes HIGH at time t5 where theobject lens is distant from the just focus point of the second layer bya distance which is half the distance at time t4, and the focus searchdrive voltage V+ (FIG. 2F) is supplied to the focus drive coil 28. Thesupply of the focus search drive voltage V+ continues until the objectlens reaches the just focus point of the second layer (point of timet6). After time t6, digital data on the second layer is reproduced inthe ordinary way.

However, the time when the object lens reaches the just focus point ofthe second layer does not necessarily coincide with t6, affected by thegravity, and the point of Time where the focus error signal ERR becomeslower than the comparison voltage V3 is determined as time t6. Note thatno affection is taken into consideration because the acceleration of theobject lens is larger than the gravity acceleration. Also note that thefocus search drive voltages V+ and V- need not equal in absolute value.In such cases, if the absolute value of the focus search drive voltageV- is M times the focus search drive voltage V+, then Equation (4)becomes ##EQU1##

FIG. 3 shows a process of control by soft ware for moving the objectlens from the just focus point of the first layer to the just focuspoint of the second layer. In upward acceleration of the object lens instep 31, since the focus search drive voltage V+ is supplied to thefocus drive coil 28 by setting the drive signal HIGH and the focus OPENsignal LOW, the object lens is accelerated upward. In step 32, after thedata of the counter is replaced with zero, the control proceeds to step33. As to whether the zero-cross detection signal FZC+ is HIGH or not instep 33, if FZC+ is HIGH, the control proceeds to step 34; however, ifFZC+ is low, the control does not proceed to step 34 until FZC+ becomesHIGH.

For downward acceleration of the object lens in step 34 after thezero-cross detection signal FZC+ becomes HIGH, the drive signal DRV isset LOW, and the focus search drive voltage V- is supplied to the focusdrive coil 28 to accelerate the object lens downward. Then the controlproceeds to step 35. In step 35 the value of the counter is held in anaddress labeled with T1 (the address labelled T* is hereinbelow calledregister T*), and in step 36 the value of the counter is again set zero.

The register T1 holds the value of a period of time counted after theobject lens is accelerated upward until the zero-cross detection signalFZC+ becomes HIGH, and the counted value is taken as the period of timeT1. The value to be counted may be time, or any other amount equivalentto time, such as clocks of the apparatus, may be selected to be held inthe register. As to whether the zero-cross detection signal FZC+ is LOWor not in step 37, if FZC+ is LOW, then the control proceeds to step 38;however, if FZC+ is HIGH, the control does not proceed to step 38 untilFZC+ becomes LOW. In step 38 the value of the counter is held inregister T2_(A).

As to whether the zero-cross detection signal FZC- is HIGH or not instep 39, if FZC- is HIGH, the control proceeds to step 40; however, ifFZC- is LOW, the control does not proceed to step 40 until FZC- becomesHIGH. In step 40 the value of the counter is held in register T2_(B),and in step 41 the average value of register T2_(A) and register, T2_(B)is held in register T2. In next step 42, a result of operation of(T1-T2)/√2 is held in register T3. In step 43, it is detected whetherthe sum of register T1 and register T3 equals the value of the counteror not. If they are equal, the control proceeds to step 44; however, ifnot, step 43 is repeated until the sum of T1 and T3 becomes equal to thevalue of the counter.

After that, as to upward acceleration of the object lens in step 44, thedrive signal DRV is set HIGH, and the focus search drive voltage V+ issupplied to the focus drive coil 28 to accelerate the object lensupward. Detection of whether the zero-cross detection signal FZC- isHIGH or not in step 45 is a chattering step, and the control proceeds tostep 46 when the zero-cross detection signal FZC- is detected to beHIGH. As to whether the zero-cross detection signal FZC- is LOW or notin step 46, if it is determined that FZC- is LOW and that focalizationon the just focus point of the second layer is established, the controlproceeds to step 47. In step 47, the focus OPEN signal is set HIGH, thuschanging the switch 18, terminating the focus jump from the first layerto the second layer, and restoring the focus servo loop.

FIGS. 4A to 4J show a timing chart for focus jump from the second layerto the first layer. At FIG. 4A, taking time T on the horizontal axis andthe lens height x on the vertical axis, the moving track of the objectlens is illustrated. As stated above, for changing focalization from thejust focus point of the second layer to the just focus point of thefirst layer, the object lens is first moved to once overshoot the justfocus point of the destined first layer, and thereafter moved back to befocalized at the just focus point of the first layer. The speed of theobject lens during these movements is shown at FIG. 4B where the lensspeed v lies on the vertical axis.

As shown at FIG. 4A, the movement from the just focus point of thesecond layer to the just focus point of the first layer starts at timet1. That is, at time t1, the control is changed from the focus servoloop control to the focus jump control. In the period of time T1 fromtime t1 to time t2, the object lens is accelerated downward. Time t2 isthe point where the S-curve of the focus error signal ERR of the firstlayer, when rising, surpasses the comparison voltage V3. After theobject lens passes time t2, it is accelerated upward, passing a point(time t3) where the object lens intersects with the just focus point ofthe first layer. The object lens is further accelerated upward, andpasses time t4 where the lens speed is zero, taking the same period oftime as T1 in which the object lens was accelerated downward. The objectlens, after passing time t4, is accelerated upward by time t5, and thencontrolled to stop at the just focus point of the first layer.

That is, as shown at FIG. 4C, digital data recorded on the second layerof the optical disk is read out as the reproduction RF signal by timet1. Since the digital data recorded on the optical disk relies on thepresence or absence of pits, the reproduction RF signal exhibits thewaveform as illustrated. In order to effect focus jump from the secondlayer to the first layer, the object lens must first be accelerateddownward. Therefore, the focus OPEN signal (FIG. 4G) is set LOW, and thedrive signal DRV (FIG. 4H) is set LOW. The focus OPEN signal shown atFIG. 4G maintains the LOW level to the end of the control for focusjump.

When the drive signal DRV becomes LOW, the object lens moves downward.Accordingly, the signal level of the reproduction RF signal is lowered.When the reproduction RF signal becomes lower than the comparisonvoltage V1, the focus OK signal FOK becomes LOW. When the signal levelof the reproduction RF signal again goes high and surpasses thecomparison voltage V1, the focus OK signal FOK becomes HIGH. This isbecause the focus jump causes the object lens to begin to move out offocalization on the second layer toward focalization on the first layer.After that, the focus jump to the first layer is completed at time t6,and digital data recorded on the second layer of the optical disk isread out.

If a focus error signal ERR (FIG. 4E) is generated in the plus side andsurpasses the comparison voltage V2, then the zero-cross detectionsignal FZC+ shown at FIG. 4H becomes HIGH. When the focus error signalin the plus side approaches 0 V and becomes smaller than the comparisonvoltage V2, the zero-cross detection signal FZC+ becomes LOW.

When the focus jump from the second layer to the first layer isperformed, the just focus point of the first layer becomes nearer thanthe just focus point of the second layer. Therefore, a focus errorsignal ERR is generated in the minus side with reference to the justfocus point of the first layer. As stated above, when the focus errorsignal ERR (FIG. 4E) surpasses the comparison voltage V3 (time t2), thezero-cross detection signal FZC- becomes HIGH, and the drive signal DRV(FIG. 4H) becomes HIGH. Therefore, the focus search drive voltage V+(FIG. 4F) is supplied to the focus drive coil 28. That is, the objectlens is accelerated upward. When the focus error signal ERR becomeslower than the comparison voltage V3, the zero-cross detection signalFZC- becomes LOW.

The drive signal DRV (FIG. 4H) again becomes HIGH at time t5 where theobject lens is distant from the just focus point of the first layer by adistance which is half the distance at time t4, and the focus searchdrive voltage V- (FIG. 4F) is supplied to the focus drive coil 28. Thesupply of the focus search drive voltage V- continues until the objectlens reaches the just focus point of the first layer (time t6). Aftertime t6, digital data on the first layer is reproduced in the ordinaryway.

FIG. 5 shows a process of control by soft ware for moving the objectlens from the just focus point of the second layer to the just focuspoint of the first layer. In downward acceleration of the object lens instep 51, since the focus search drive voltage V- is supplied to thefocus drive coil 28 by setting the drive signal DRV LOW and the focusOPEN signal LOW, the object lens is accelerated downward. In step 52,after the data of the counter is replaced with zero, the controlproceeds to step 33. As to whether the zero-cross detection signal FZC-is HIGH or not in step 33, if FZC- is HIGH, the control proceeds to step54; however, if FZC- is low, the control does not proceed to step 34until FZC- becomes HIGH.

For upward acceleration of the object lens in step 54 after thezero-cross detection signal FZC- becomes HIGH, the drive signal DRV isset HIGH, and the focus search drive voltage V+ is supplied to the focusdrive coil 28 to accelerate the object lens upward. Then the controlproceeds to step 55. In step 55 the value of the counter is held inregister T1, and in step 36 the value of the counter is again set zero.That is, register T1 holds the time in which the object lens isaccelerated upward. As to whether the zero-cross detection signal FZC-is LOW or not in step 57, if FZC- is LOW, then the control proceeds tostep 58; however, if FZC- is HIGH, the control does not proceed to step58 until FZC- becomes LOW. In step 58 the value of the counter is heldin register T2_(A).

As to whether the zero-cross detection signal FZC+ is HIGH or not instep 59, if FZC+ is HIGH, the control proceeds to step 60; however, ifFZC+ is LOW, the control does not proceed to step 60 until FZC+ becomesHIGH. In step 60 the value of the counter is held in register T2_(B),and in step 41 the average value of register T2_(A) and register T2_(B)is held in register T2. In next step 62, a result of operation of(T1-T2)/√2 is held in register T3. In step 63, it is detected whetherthe sum of register T1 and register T3 equals the value of the counteror not. If they are equal, the control proceeds to step 64; however, ifnot, step 63 is repeated until the sum of T1 and T3 becomes equal to thevalue of the counter.

After that, as to downward acceleration of the object lens in step 64,the drive signal DRV is set LOW, and the focus search drive voltage V-is supplied to the focus drive coil 28 to accelerate the object lensdownward. Detection of whether the zero-cross detection signal FZC- isHIGH or not in step 65 is a chattering step, and the control proceeds tostep 66 when the zero-cross detection signal FZC+ is detected to beHIGH. As to whether the zero-cross detection signal FZC+ is LOW or notin step 66, if it is determined that FZC+ is LOW and that focalizationon the just focus point of the first layer is established, the controlproceeds to step 67. In step 67, the focus OPEN signal is set HIGH, thuschanging the switch 18, terminating the focus jump from the second layerto the first layer, and restoring the focus servo loop.

FIGS. 6A to 6J show a timing chart for focus jump from the first layerto the third layer as an example of focus jump of two or more layers. AtFIG. 6A, taking time T on the horizontal axis and the lens height x onthe vertical axis, the moving track of the object lens is illustrated.As stated above, for changing focalization from the just focus point ofthe first layer to the just focus point of the third layer, the objectlens is first moved to once overshoot the just focus point of thedestined third layer beyond the just focus point of the second layer andfurther overshoot the just focus point of the fourth layer, andthereafter moved back to be focalized at the just focus point of thethird layer. The speed of the object lens during these movements isshown at FIG. 6B where the lens speed v lies on the vertical axis.

As shown at FIG. 6A, the movement from the just focus point of the firstlayer to the just focus point of the third layer starts at time t1. Thatis, at time t1, the control is changed from the focus servo loop controlto the focus jump control. In the period of time T1 from time t1 to timet2, the object lens is accelerated upward. Time t2 is the point wherethe S-curve of the focus error signal ERR of the first layer, whenrising, surpasses the comparison voltage V2. After the object lenspasses time t2, it is accelerated downward, passing a point (time t3)where the object lens intersects with the just focus point of the thirdlayer. The object lens is further accelerated downward, and passes timet4 where the lens speed is zero, taking the same period of time as T1 inwhich the object lens was accelerated upward. The object lens, afterpassing time t4, is accelerated downward by time t5, and then controlledto stop at the just focus point of the third layer.

That is, as shown at FIG. 6C, digital data recorded on the first layerof the optical disk is read out as the reproduction RF signal by timet1. Since the digital data recorded on the optical disk relies on thepresence or absence of pits, the reproduction RF signal exhibits thewaveform as illustrated. In order to effect focus jump from the firstlayer to the third layer, the object lens must first be acceleratedupward. Therefore, the focus OPEN signal (FIG. 6G) is set LOW, and thedrive signal DRV (FIG. 6H) is set HIGH. The focus OPEN signal shown atFIG. 6G maintains the LOW level to the end of the control for focusjump.

When the drive signal DRV becomes HIGH, the object lens moves upward.Accordingly, the signal level of the reproduction RF signal is lowered.When the reproduction RF signal becomes lower than the comparisonvoltage V1, the focus OK signal FOK becomes LOW. When the signal levelof the reproduction RF signal again goes high and surpasses thecomparison voltage V1, the focus OK signal FOK becomes HIGH. This isbecause the focus jump causes the object lens to begin to move out offocalization on the first layer toward focalization on the second layer.In this example, since the focus jump is to be effected from the firstlayer to the third layer, as shown at FIG. 6C, focalization is movedfrom the first layer to the third layer, from the second layer to thethird layer, from the third layer to the fourth layer, and finally tothe destined third layer, passing the fourth layer another time. Afterthat, the focus jump to the first layer is completed at time t6, anddigital data recorded on the third layer of the optical disk is readout.

If a focus error signal ERR (FIG. 6E) is generated in the minus side andsurpasses the comparison voltage V3, then the zero-cross detectionsignal FZC- shown at FIG. 6J becomes HIGH. When the focus error signalin the minus side approaches 0 V and becomes smaller than the comparisonvoltage V3, the zero-cross detection signal FZC- becomes LOW.

When the focus jump from the first layer to the third layer isperformed, the just focus point of the second layer becomes nearer thanthe just focus point of the first layer. Therefore, an S-curve is of afocus error signal ERR is generated in the minus side with reference tothe just focus point of the first layer. After that, a plus-side S-curvebased on the second layer is generated, and a minus-side S-curve is alsogenerated. As illustrated, when the object lens passes the just focuspoint while moving upward, an S-curve of the focus error signal ERRfirst appears in the plus side and next in the minus side. When theobject lens passes the just focus point while moving downward, anS-curve first appears in the minus side and next in the plus side.

In this manner, the process of focus jump of two or more layers isexactly the same as the process of focus jump of one layer except thatthe time T1 is extended to the rising of the S-curve of a focus errorsignal ERR of a destined layer. The maximum height for movement of theobject lens is within the range where the object lens does not hit theoptical disk.

Although the embodiment has been described setting the time from thestart of acceleration of the object lens to the rising of the S-curve ofthe focus error signal ERR of a destined layer as the time T1 for thefirst acceleration of the object lens, the time T1 may be set otherwiseprovided the object lens can be accelerated such that focalization ofthe object lens moves beyond the just focus point of the destined layer.

Moreover, although the embodiment of the soft-ware processing accordingto the invention employs the address labelled T* as the register T*, itis also possible to use the label T* itself as the register T*.

According to the invention, focalization to a destined layer can beestablished in a short time as compared the method of resuming a focussearch operation after canceling the focus servo control.

Also, the invention can reliably catch the just focus point of adestined layer even with a variety in distance between layers becausefocus servo control is performed after moving the focalization beyondthe just focus point of the destined layer, unlike the method of upwardor downward acceleration for a predetermined time.

Moreover, the invention can perform a reliable servo control afterclosing the focus servo loop because the speed of the object lens at thejust focus point of a destined layer is approximately zero.

Having described specific preferred embodiments of the present inventionwith reference to the accompanying drawings, it is to be understood thatthe invention is not limited to those precise embodiments, and thatvarious changes and modifications may be effected therein by one skilledin the art without departing from the scope or the spirit of theinvention as defined in the appended claims.

What is claimed is:
 1. A disk reproducing method for reproducing datarecorded on a plurality of layers of an optical disk, comprising:a firststep for accelerating an object lens in a first direction; a second stepfor detecting that a focus error signal surpasses a first voltage andfor accelerating the object lens in a second direction different fromsaid first direction; a third step for detecting that the object lenshas moved for a predetermined duration of time and for accelerating theobject lens in said first direction; and a fourth step for detectingthat the focus error signal surpasses a second voltage and for startingfocus servo control.
 2. The disk reproducing method according to claim1, wherein the acceleration in said first direction and the accelerationin said second direction are opposite in direction and substantiallyequal in absolute value.
 3. The disk reproducing method according toclaim 2, wherein said third step further comprises: a step for obtaininga duration of time T1 of acceleration in said first direction by saidfirst step; and a step for obtaining a duration of time T2 since theacceleration in said second direction by said second step starts untilthe object lens passes over a focalized point.
 4. The disk reproducingmethod according to claim 3, wherein said third step further comprises astep for obtaining from said T1 and T2 a duration of time from a pointof time where the object lens moves from the position most distant fromthe focalized point to a point of time where the object lens passes overa position distant from the focalized point by one half the distance ofthe most distant position from the focalized point.
 5. The diskreproducing method according to claim 1, further comprising a step ofcomparing a reproduced RF signal with a third voltage.